This invention relates to apparatus for monitoring current flow in a power supply line. More particularly, it is concerned with apparatus for monitoring the flow of current in the ground conductor of a polyphase AC electrical power system.
It is a common practice to monitor current flow in the ground conductor of a three-phase electrical system and to trip circuit breakers protecting the load connected to the system when the current flow exceeds certain predetermined limits. It is often desirable to introduce a delay in order to avoid nuisance tripping due to transient conditions. The delay period may be variable to permit adjustments for particular situations.
One form of apparatus for monitoring the current in a ground conductor employs a fault current sensor which includes a normally open magnetic reed switch. The magnetic flux produced by a fault current in the conductor causes the switch to close and open at twice the frequency of the current. While the switch is closed, the capacitance in a resistance-capacitance circuit is charged by a constant current source. The capacitance is connected to a switching transistor, and when the voltage across the capacitance is sufficient, the transistor turns on causing circuit breaker apparatus coupled thereto to trip. The time required to charge the capacitance may be varied by changing the value of the resistance, thereby varying the delay period.
With the foregoing apparatus it is difficult to obtain an accurately controlled delay period. There is a problem in matching the values of the resistance, the capacitance, and the source of constant current. In addition, individual component values may vary over the operating temperature range encountered. Since the portions of a cycle during which the contacts of the reed switch are closed (the duty factor) depends upon the magnitude of the fault current and because of contact bouncing, the length of time the contacts close varies. Calibration of the delay period is thus a serious problem. Testing and calibration in the field are particularly difficult.